The present invention relates to a system for identifying stereo and dual audio signals according to the broadcast method used to transmit the audio signals. In particular, the present invention identifies stereo and dual audio signals which are broadcast using two carrier frequencies.
Conventional circuits for identifying stereo and dual audio signals are complicated. Accordingly, conventional circuits occupy an unacceptably large amount of space when integrated within a semiconductor chip. Earlier attempts to simplify the structure and function of circuits identifying stereo and dual audio signals in order to reduce the amount of required chip space have resulted in unreliable circuit performance.
The circuit block diagram shown in FIG. 1 is exemplary of conventional stereo and dual audio identifying circuits. The circuit in FIG. 1 is characterized by the presence of two phase detectors and two associated 90.degree. phase shifters. In FIG. 1, a second intermediate frequency (IF) signal is applied to an FM detector 10. The FM-detected output from FM detector 10 is then applied to an AM detector 12 through bandpass filter 11. AM detector 12 detects the envelope of the bandpass filtered signal, and applies the result to first inputs of phase comparators 13 and 18, and to phase detectors 17 and 22. Reference signals from voltage-controlled oscillators (VCOs) 15 and 20 are respectively applied to second inputs of phase comparators 13 and 18. The VCO reference signals are also respectively applied to second inputs of phase detectors 17 and 22 through 90.degree. phase shifters 16 and 21.
VCO 15 is driven by the output of phase comparator 13 which is filtered by low-pass filter 14 before being applied to VCO 15. Similarly, VCO 20 is driven by the output of phase comparator 18 which is filtered by low-pass filter 19. Typically, VCO 15 generates a frequency of 150 Hz, and VCO 20 generates a frequency of 276 Hz to match the synchronization frequencies of the stereo and dual audio signals.
The outputs of phase detectors 17 and 22 are applied to comparator 23 as first and second inputs respectively. The output of comparator 23 serves as a mode select signal which discriminates between two frequencies used in the broadcast format for the stereo and dual audio signals.
This conventional circuit generally performs well, but the number, type and required arrangement of the numerous subcircuits makes compact integration impossible. Given the increasing requirements for miniaturization, the large chip space requirements of the conventional circuit are unacceptable.